docs: def save(self):
          """ Generate a random username before falling back to parent signup form """
          while True:
              username = sha1(str(random.random()).encode('utf-8')).hexdigest()[:5]
              try:
                  get_user_model().objects.get(username__iexact=username)
              except get_user_model().DoesNotExist: break
  
          self.cleaned_data['username'] = username
          return super(SignupFormOnlyEmail, self).save()